javascript - Default value for parameter expect function

I want to add default value for function parameter expect to receive a function.

Look at the following example:

/** 
* @param {function} callback
* @param {String} name
*/
function example(callback, name){
    console.log('Hello' + name + 'from parent function');
    callback();
}



// implementation
example(null, "Mickey");

I want to make the callback as optional parameter.

Answer

In JavaScript you typically test whether the parameter is null or undefined and provide a default. More succinctly you can use ||:

callback = callback || (some default value);

Using || is looser because it will also treat anything that's "falsy" in JavaScript as missing, including false, an empty string and zero. So it's not useful for optional boolean parameters.

In your case, your default value is a function:

function example(opt_callback, name) {
  var callback = opt_callback || function() {};
  console.log('Hello' + name + 'from parent function');

  callback();
}

Here the default function provides nothing, but you could add some default behavior to the default function here.

Naming optional arguments with opt_ is a practice recommended by the Google JavaScript Style Guide and Google Closure Compiler. I think it makes code more readable, but maybe I have just gotten used to it.

Unless there are other overriding readability reasons, it is good to put optional arguments at the end of the parameter list, for example:

function example(name, opt_callback) {
  ...

That way callers can succinctly call your function with just one argument, for example example('Mickey'), because the default value of unspecified arguments in JavaScript is undefined.